As high-speed metal cutting at a feed per one edge exceeding 0.3 mm becomes prevailing, conventional hard-coated tools have become insufficient in oxidation resistance, wear resistance, peel resistance, seizure resistance, impact resistance, breakage resistance, etc. of hard coatings. Accordingly, various technologies have been proposed to improve oxidation resistance, wear resistance, peel resistance, seizure resistance, impact resistance, breakage resistance, etc. of the hard coatings.
JP 2003-225807 A discloses a cutting tool having a hard coating layer exhibiting excellent wear resistance in high-speed cutting, the hard coating layer being formed by physically depositing a composite nitride of Ti and Y to an average thickness of 1-15 μm on a substrate of tungsten carbide-based cemented carbide or titanium carbonitride cermet, the hard coating layer having maximum-Y-component points (minimum-Ti-component points) and Y-component-free points (TiN points) alternately at a predetermined interval in a layer thickness direction, the concentration of the Y component changing smoothly between the maximum-Y-component point and the Y-component-free point, the maximum-Y-component points having a composition represented by the formula of (Ti1-xYx)N, wherein x is 0.05-0.15 by atomic ratio, an interval between the adjacent maximum-Y-component point and the Y-component-free point being 0.01-0.1 μm. Japanese Patent 3,460,288 discloses a wear-resistant coated member comprising a substrate and a hard coating formed on its surface, the hard coating being formed by layers of nitrides, oxides, carbides, carbonitrides or borides of 2 or more elements selected from the group consisting of metal elements of Groups 4a, 5a and 6a, Al and Si, such that their compositions change continuously at a period of 0.4 nm to 50 nm to a total thickness of 0.5-10 μm. The hard coatings of JP 2003-225807 A and Japanese Patent 3,460,288 are multi-layer films with repeatedly changing concentrations or compositions. However, because any of the above technologies uses only an arc-discharge ion plating method, edge, they do not necessarily have satisfactory seizure resistance in the cutting of steel, etc., on which seizure tends to occur.
The seizure resistance of hard coatings depends on their lubrication. Hard-coated tools satisfactorily usable under severe cutting conditions cannot be obtained without improving the lubrication, impact resistance and breakage resistance of hard coatings with no damage to adhesion to substrates, hardness, oxidation resistance, wear resistance, thermal resistance, etc. In addition, taking environment into consideration, demand is mounting on tools for use in dry cutting without using cutting oils containing Cl, S, P, etc., for instance, even in the cutting of die-casting steel, called hard-to-cut materials.
As a cutting tool having a hard coating with improved lubrication, JP 5-239618 A discloses a machining tool with a coating having high wear resistance and lubrication, the coating comprising at least one element selected from the group consisting of oxygen, sulfur, selenium and tellurium, and at least one element selected from the group consisting of vanadium, niobium, tantalum, chromium, molybdenum and tungsten, and containing molybdenum disulfide for imparting lubrication. JP 11-509580 A discloses a method for forming a high-lubrication, hard coating comprising, for instance, molybdenum disulfide and TiN on a cutting tool, using a sputtering-ion plating system comprising a first target made of a metal sulfide (for instance, molybdenum disulfide), and a second target made of at least one metal selected from the group consisting of titanium, vanadium, chromium, zirconium, niobium, molybdenum, tantalum, hafnium and tungsten. However, these technologies do not provide hard coatings with sufficient adhesion and hardness, failing to sufficiently improve the wear resistance of cutting tools.
JP 8-127863 A discloses a wear-resistant, hard laminate coating comprising as main components at least one element selected from the group consisting of elements of Groups IVa, Va and VIa in the Periodic Table, Al, Si and B, and at least one element selected from the group consisting of B, C, N and O, the coating having at least 2 types of compound layers having different compositions and composition-changing layers, whose element compositions change in a thickness direction between the compound layers, the compound layers and the composition—changing layer being periodically laminated, and a crystal lattice being continuous over one period or more between the layers. Japanese Patent 3,416,938 discloses a multi-layer, hard coating for cutting tools, etc., in which compound layers (for instance, TiN) and layers with element compositions changing in a thickness direction (for instance, TiAlN) are alternately laminated, a crystal lattice being continuous with strain in said composition-changing layer.
JP 2001-293601 A discloses a cutting tool having a wear-resistant hard coating formed on a tool substrate, the coating comprising as a main component a nitride or carbonitride of at least one element selected from the group consisting of elements of Groups 4a, 5a and 6a in the Periodic Table and Al, said substrate being made of at least one selected from the group consisting of WC-based cemented carbide, cermet, silicon carbide, silicon nitride, aluminum nitride, alumina, boron carbide, sintered aluminum oxide-titanium carbide, high-speed steel, die steel and stainless steel, said wear-resistant, hard coating containing at least one type of fine, hard particles selected from the group consisting of B4C, BN, TiB2, TiB, TiC, WC, SiC, SiNx (x=0.5-1.33) and Al203.
However, any of the hard coatings described in the above references fails to meet the demand of having sufficient lubrication, peel resistance, impact resistance and breakage resistance capable of withstanding dry cutting conditions while maintaining oxidation resistance and wear resistance.